Features defined in a semiconductor may be characterized by a critical dimension CD (i.e., a target critical dimension), which is typically specified by a chip designer. A target critical dimension specification provides upper and lower limits of the critical dimension. The features formed in the photoresist (i.e., an actual dimension) must be within the target critical dimension specification.
Unfortunately, the various systems and components used in an image transfer process to form features in a photoresist on a semiconductor wafer provide a multiplicity of error sources across an image field. Further, an error at one point in the image field may be different than an error at another point in the image field.
For example, the image transfer system, which includes an illumination source and a lens, will introduce errors into the image transfer process. One source of errors is the illumination source, which may not provide a uniform output intensity, thus causing the light energy impinging on the photoresist surface to vary. The local degree of partial coherence, which is difficult to control, is another source of error in the image transfer process. Lens aberrations provide yet another source or error. Still another source of error is the product reticle or mask, which provides the feature pattern for the photoresist. Due to manufacturing device limitations, dimensions of the features defined on the reticle deviate from target dimensions. Many image transfer systems use reduction optics with a magnification ratio, M, such that the pitch (P) of reticle features is imaged into the photoresist with a pitch of P/M. The effective width (W) of a reticle feature is W/M. For purposes of this disclosure, M=1, but the present invention is not limited to M=1. Consequently, the actual dimensions of features formed in the photoresist will likely deviate from the target critical dimension and may fall outside the limits of the target critical dimension specification.
Commonly, reticles include lithographic control features (LCFs) which are used to determine the amount of deviation between target and actual dimensions. When features are formed in the photoresist, a resulting LCF formed in the photoresist may be measured to determine a deviation between its actual dimension and the target critical dimension. The deviation information is useful to determine if that resulting LCF feature has an actual dimension that is within the limits defined by the target critical dimension specification. Unfortunately, the actual dimension of that resulting LCF does not directly correlate with the actual dimensions of other features defined by the reticle pattern and formed in the photoresist. This occurs because the errors may vary across the image field. Thus, measuring the dimensional deviation of the LCF does not necessarily provide useful information on the actual dimensional deviation for non-LCF features and whether they fall within the specification limits. Clearly, the information obtained via the LCF critical dimension deviation is generally inadequate to control or adjust downstream processing steps and devices when significant reticle errors and image transfer errors are present--as they always are.
There thus exists a need in the art for a method of controlling the critical dimension of features patterned in a photoresist on a semiconductor wafer that overcomes the above-described shortcomings of the prior art.